Anchoring systems forming the foundations of buildings or other large structures are commonly used where adequate bearing capacity cannot be found to support the structural loads. Helical piles, which have a pile shaft with one or more spiral helical plates affixed thereto, can be rotated into the ground to support structures, providing a versatile and efficient alternative to conventional pile systems. It is well known that a pile's capacity is highly dependent upon the pile's configuration and the surrounding soil conditions, and that axial capacity of each pile in both tension and compression must be tested with significant accuracy prior to commencing construction of the supported structure. Such tests need to account for the impact of the number and size of plates upon the pile's capacity. Thus, there is a desire for precise and reliable apparatus and methodologies for testing load capacity of helical piles.
Conventional “top-down” load testing methods typically involve driving a test pile into the ground, incrementally applying pressure directly to the top of the pile (using a hydraulic jack or ram), and then measuring axial (e.g. upward and/or downward) movement of the pile. Conventional tests can be used to determine the maximum pressure required to pull the pile from the soil and the maximum load that can be supported by the pile without failure.
Some load testing methods incorporate the use of a “reaction pile system”, which comprises the positioning of two “reaction piles” adjacent to the test pile, each reaction pile supporting a cross-beam braced against the test pile to monitor the pressure supplied by the jack and the associated displacement of the test pile.
Conventional “top-down” testing systems, however, require substantial loads to be applied to the test the piles (e.g., thousands of tonnes), resulting in extremely dangerous, costly, and time-consuming testing procedures. Further, at least two separate tests must be performed to determine the upward resistance and end-bearing capacity for each test pile.
U.S. Pat. Nos. 4,614,110 and 5,576,494 describe the Osterberg Cell®, or O-cell®, which is a load-generating cell designed to reduce the loads required in load capacity testing. The O-cell® is a hydraulically-driven cell installed at or near the bottom of drilled shafts, bored piles, driven piles, or other similarly constructed pile foundations. When the cell is pressurized, it generates loads bi-directionally along the shaft of the pile, that is—it expands to simultaneously apply force both upwardly and downwardly. The upward “pullout” force from the top of the cell is resisted by the shaft of the pile (i.e. providing the “skin-friction”) and the downward force from the bottom of the cell is resisted by the interaction between the soil and the pile (i.e. determining the “end-bearing capacity” or “resistance”). Because the O-cell® is irretrievably instrumented into the shaft of the pile, however, each cell must be sacrificed and cannot be reused. Further, because of its positioning within the pile shaft, the overall strength of the pile shaft is reduced, preventing it from withstanding the higher torque necessary to rotate the piles into the soil. As such, the O-cell® is not readily appropriate for use with helical piles.
U.S. Pat. No. 8,517,640 describes an expandable bi-directional static load capacity testing system that is adapted for use with helical piles. The system involves dividing the helical pile into shaft and “toe” sections, and positioning a jack-like apparatus (e.g. such as the O-cell®) between the shaft and toe sections, and positioning first and second helical plates above the jack-like apparatus. This system, however, still requires that the jack-like apparatus be sacrificed and that two separate tests must be performed to obtain accurate shaft resistance and end bearing capacity. Further, where multiple helices are desired, the system requires that at least two helices are positioned above and one below the O-cell®.
There is a need for an improved testing system that can more accurately determine the load capacity of helical piles, and particularly of helical piles having a plurality of helical plates. It is desired that such a testing system could provide accurate resistance and end-capacity information for each plate(s). It is further desired that much of the system could be salvaged for reuse.